This invention relates in general to ultrasound systems, and in particular to an ultrasound system for measuring fluid volume flow.
Volume flow measurements may be important for various medical diagnosis. Volume flow indicates blockage in blood vessels and the performance of diseased or transplanted organs. For example, changes in the blood flow out of a kidney over time may be determined. Other examples of clinical application of volume flow measurements include: blood flow through shunts, blood flow to or from transplanted or diseased organs, umbilical cord and uterine artery flow, flow through various arteries and vessels, the blood flow in the brachial artery before and after artificially induced ischemia, flow through mitral aortic tricuspid and pulmonic valves, and others.
Ultrasound systems have been used to estimate volume flow. For example, a mean velocity estimate for a small sample volume inside a vessel is obtained from spectral Doppler information. An angle of flow is estimated from a user input angle. The user also manually outlines the vessel's cross section to obtain an estimate of area. The mean velocity, area and the appropriate trigonometric function of the Doppler angle are multiplied to obtain a flow estimate. However, the various manual tracings and estimations are laborious and prone to inaccuracies due to human error. Furthermore, obtaining the mean velocity from one sample volume may not accurately represent the true mean velocity across the entire vessel.
In another ultrasound technique for measuring volume flow, a high spatial resolution image is used to measure the flow profile across a vessel. The individual estimates of flow from each volume cell within a vessel are summed together to obtain the total volume flow. However, due to non-ideal ultrasound beam profiles, the information from one volume cell may duplicate, in part, another volume cell. Furthermore, this technique assumes that flow is parallel to the vessel or requires user estimation of the flow angle.
In yet another ultrasound technique to obtain volume flow, the velocity profile across a vessel is assumed to correspond to a particular function, such as a parabolic or plug profile. A single velocity estimate is obtained at the center of the vessel and used to estimate volume flow. The area of the vessel is calculated either manually or assumed to be circular. However, the area measurement is prone to human or estimation errors, and the actual flow profiles of fluids within a vessel may not match the parabolic or plug functions. Furthermore, as discussed above, the flow angle is manually entered, making the volume flow calculation laborious and error prone.
In yet another ultrasound technique for measuring volume flow, a cross section of a vessel located within a sample volume is insonified using a C-scan. See Hottinger U.S. Pat. No. 4,067,236. Therefore, ultrasound information is obtained from a plane parallel to the face of the transducer. In order to obtain the C-scan information, a fixed one or two-element transducer or a two-dimensional array transducer is used. The first moment of the C-scan information is calculated, eliminating the need to measure the area of the vessel. Measuring data in a plane parallel to the face of the transducer also eliminates the need to measure the flow direction. However, this technique does not accurately estimate volume flow in vessels that run parallel to the face of the transducer. Additionally, specialized transducers are required.